Life Cycle Analysis and Temporal Distributions of Emissions: Limitations of Product-Centered Emission Analyses

Description:

Abstract
Although the product-centered focus of life cycle analysis has been one of its strengths, this
analytical perspective embeds assumptions that may conflict with the realities of the problems to
which it is applied. This paper demonstrates, through a series of mathematical derivations, that
all the products in use, rather than a single product, frequently should be the appropriate unit of
analysis. Such a "fleet-centered" approach supplies a richer perspective on the comparative
emissions burdens generated by alternative products, as well as eliminating certain simplifying
assumptions imposed upon the analyst by a product-centered approach.
A sample numerical case, examining the comparative emissions of steel-intensive and aluminum
intensive automobiles, is presented to contrast the results of the two approaches. The
fleet-centered analysis shows that the "crossover time" (i.e., the time required before the fuel
economy benefits of the lighter aluminum vehicle offset the energy-intensity of the processes
used to manufacture the aluminum in the first place) can be dramatically longer than that
predicted by conventional life cycle analyses.
The fleet-centered perspective explicitly introduces the notion of time as a critical element of
comparative life cycle analyses and raises important questions about the role of the analyst in
selecting the appropriate time horizon for analysis. Moreover, with the introduction of time as an
appropriate dimension to life cycle analysis, the influences of effects distributed over time can be
more naturally and consistently treated.